This application relates to a return tube for use with a ball screw assembly. More particularly, the present invention discloses a relationship between the minor legs of the return tube and the apertures to receive the minor legs formed in the nut of the ball screw assembly.
Ball screw assemblies are used in several applications where extremely accurate positioning is desired. Essentially, a screw member having a groove at an external surface is received within an internal bore of a nut, wherein the internal bore has a mating internal groove extending between first and second ends. Ball bearings are received between the grooves in the screw and the nut. The screw or the nut is rotated relative to the other, and the non-rotated member moves axially relative to the rotated member. As the nut moves relative to the screw, the ball bearings move within the grooves. A return tube is positioned at one end of the internal groove within the nut and communicates balls from that end to the other end of the groove in the nut. Thus, as the member moves, the ball bearings move within the helical grooves between the nut and the screw and are returned from a first end of the internal groove in the nut to the second end through a return tube.
In many prior art ball screw assemblies, ball bearings leaving the groove were subjected to an abrupt directional change as they entered the return tube. This was inefficient and resulted in noise and heat loss from the assembly.
An improved ball screw assembly is known in which the return tube comprises a major leg extending along an axis parallel to the axis of the screw and a pair of minor legs extending at an acute angle relative to the major leg, and communicating with the first and second ends of the internal groove at an angle parallel to the helix angle, and tangent to the helical groove at the end. Since the minor leg of the return tube is tangent to the helical groove, the ball bearing does not encounter any abrupt direction change and thus exits the groove and enters the return tube without the previously encountered noise or heat losses. Essentially, the minor leg is positioned such that a ball bearing exiting the end of the internal groove continues moving in the same direction as it enters the return tube. One such device is disclosed in U.S. Pat. No. 4,953,419, issued to the inventor of the present invention.
One potential limitation on the above-disclosed device is that the return tube normally has to be formed of two pieces in order for the two minor legs to each be inserted into the apertures in the nut. The helix angles at the first and second end of the internal groove tend to be in opposite directions, and thus the minor legs extend in opposed directions. Since the two minor legs extend in opposed directions, they cannot normally each be inserted into respective apertures if the return tube is formed of a single piece.
In some applications it is desirable to have one-piece return tube. A two-piece return tube may not be as structurally sound as a one-piece return tube. Further, the use of the two-piece tube requires additional steps in assembling the ball screw assembly.
For this reason it would be desirable to have a ball screw assembly in which minor legs enter the helical groove at an angle parallel to the helix angle and tangent to the helical groove at the ends of the groove, and wherein the return tube is a one-piece item.